The present invention relates generally to an electromagnetic (EM) tracking system, and more particularly to, an EM position and orientation tracking system operating at an ultra-low frequency.
Many medical procedures involve a medical instrument, such as a drill, catheter, scalpel, scope, stent or other tool. In some cases, a medical imaging or video system may be used to provide positioning information for the instrument, as well as visualization of an interior of a patient. Typically, during the course of a procedure, an instrument is guided by continuously obtaining and viewing x-ray images that show the current location of the instrument along with a portion of the patient's anatomy in a region of interest. However, because repeated exposure to x-ray radiation is harmful to medical personnel that perform image guided procedures on a daily basis, many navigation systems have been proposed that attempt to reduce exposure to x-ray radiation during the course of a medical procedure.
For example, electromagnetically tracking the position and orientation of medical instruments during a medical procedure is used as a way to decrease exposure to x-ray radiation by decreasing the number of x-ray images acquired during a medical procedure. Typically, an electromagnetic tracking system employs a transmitter and a receiver. The transmitter emits at least one signal at a frequency that is picked up by the receiver. The signal(s) from the transmitter is/are received at the receiver and the tracking system calculates position and orientation information for the medical instrument with respect to the patient or with respect to a reference coordinate system. During a medical procedure, a medical practitioner may refer to the tracking system to ascertain the position and orientation of the medical instrument when the instrument is not within the practitioner's line of sight.
The tracking or navigation system allows the medical practitioner to visualize the patient's anatomy and track the position and orientation of the instrument. The medical practitioner may then use the tracking system to determine when the instrument is positioned in a desired location. Thus, the medical practitioner may locate and operate on a desired or injured area while avoiding other structures with less invasive medical procedures.
EM position and orientation tracking systems typically contain one or more transmitters, one or more receivers, electronics to measure the mutual inductances between the transmitters and receivers, and a mechanism to calculate the position and orientation of the receivers with the respect to the transmitters.
EM position and orientation tracking systems commonly employ the industry-standard coil architecture (ISCA). ISCA uses three co-located orthogonal quasi-dipole transmitters and three co-located quasi-dipole receivers. Other systems may use three large, non-dipole, non-collocated transmitters with three collocated quasi-dipole receivers. Another tracking system architecture uses six or more transmitters spread out in space and one or more quasi-dipole receivers. Alternatively, a single quasi-dipole transmitter may be used with six or more receivers spread out in space. Alternatively still, a tracking system may use a single transmitter and a single receiver.
Conventional alternating-current (AC) based EM position and orientation tracking systems generally operate at frequencies between 8 kHz and 40 kHz. More specifically, 14 kHz is a common frequency. Other conventional EM position and orientation tracking systems, such as those described in U.S. Pat. Nos. 4,849,692 and 4,945,305, employ pulsed-direct-current “pulsed-DC” magnetic fields. These pulsed-DC tracking systems typically operate at lower frequencies than AC based EM position and orientation tracking systems.
The lower the operating frequency of an EM position and orientation tracking system the slower or less frequently measurement (e.g., position and orientation) updates occur. In addition, as the low operating frequency approaches the typically frequency of the ubiquitous AC electrical power that is supplied by the typical power utilities, typically 50-60 Hz, the magnetic fields and low harmonics of the utility power distorts the measurements.
In addition, for very electrically conductive materials, the conventional low frequencies are not low enough to make the skin depth large compared to the material's typical thickness, so these trackers experience field distortion and inaccurate tracking.